When high load rolling causing a line load of 1.0 t/mm or more, especially a line load of 2.0 t/mm or more is carried out in one of the latter-stage stands in a row of finishing mills disposed in a manufacturing line of a hot-rolled steel sheet, a surface of a rolling roll tends to be rough due to increased surface pressure of the rolling roll. This surface roughness affects coolability and surface properties of a rolled material; therefore, in order to cool the rolled material uniformly in the entire width direction thereof and manufacture a hot-rolled steel sheet having excellent surface properties, some measures need to be taken to inhibit and resolve the surface roughness of the rolling roll.
It is known to use a lubricant as a means to inhibit the surface roughness, and it is known to perform online roll grinding as a means to resolve the surface roughness. Using the lubricant enables reduction of a rolling load and thereby reduction of a surface pressure of the rolling roll. Therefore, the surface roughness can be inhibited. However, in the high load rolling, it is difficult to fully reduce the rolling load only with the lubricant; and if the high load rolling using the lubricant is carried out for a long period of time, inhibition of the surface roughness is likely to be insufficient. On the other hand, if online roll grinding is carried out, it is possible to make the surface of the rolling roll a smooth curved face by grinding the roughened surface of the rolling roll. However, since there is a large wear amount of the rolling roll in the high load rolling, simply carrying out the online roll grinding is likely to lead to such problems as increase in the costs of the rolling roll due to increase in the grinding amount thereof, and degradation of the productivity due to long grinding time.
Therefore, in order to uniformly cool a rolled material when the high load rolling is carried out and to manufacture a hot-rolled steel sheet having excellent surface properties, it is necessary to employ both a lubricant and online roll grinding. However, if a large amount of lubricant remains on a work roll surface, grinding unevenness occurs during grinding by an online roll grinding device. When there is grinding unevenness, it is difficult to grind the work roll into a target roll roughness, and therefore it is likely to be difficult to uniformly cool a material rolled in the high load rolling, and to improve the surface properties of the rolled material.
As a technique related to such a hot rolling line, Patent Document 1 for example discloses a roll lubrication method in which to supply a rolling lubricant to work rolls during hot rolling, wherein a rolling oil mixed with water by an oil-water mixing device is sprayed at the work rolls, while the work rolls are ground by an online roll grinding device. Further, Patent Document 2 discloses a hot rolling method in which to start applying a rolling lubricant oil to work rolls after a front end portion of a material to be rolled is fed through the work rolls; carryout lubrication rolling until rolling of a back end portion of the material to be rolled is completed; and remove the rolling lubricant oil adhered to the work rolls at the time of rolling the preceding material to be rolled, during the time between completion of the rolling of the preceding material to be rolled and entry of a following material to be rolled through the work rolls.